Terrible fuel economy

[milfdestroyer]

Hey guys newbie here. Just picked up a 1986 320i and it being a little funny! likes to idle super high, like 2000 on a 80f day then ten minutes to get down to 1000 but stays there, never gets to 700 or 800. It has a manual idle air circuit and no O2 sensor or cat. It burns 30L of fuel in 180km doing normal mixed highway/city driving. That seems terrible, like obscene. Any one else have symptoms like this? any idea where I should start? Worth mentioning though, it drives amazing! smooth as butter! No issues at all, just burns gross amounts of fuel.

I thought all fuel injected cars had a Lambda sensor, Is it normal not to have a Oxygen sensor?


Thanks in advance

[Speedtouch]

Welcome to the Zone! 17-odd mpg does sound rather poor - they should achieve around 26mpg on average, with normal mixed driving.

I would check the usual service items are up to scratch, and also check the water-heated air slide valve is operating - they have a tendency to seize up, causing the high idle:
https://www.e30zone.net/e30wiki/index.p ... e=Jetronic

[flybynite]

Best to ask in the E30 Technical Help section as questions often get missed.

viewforum.php?f=2

There have been a good few "idle issues" threads that are good to look through if you search the forum (like this one)

viewtopic.php?f=2&t=280891

viewtopic.php?f=2&t=280788

viewtopic.php?f=2&t=279001

The LE Jetronic had very little of what you would associate with a modern fuel injection system

[longanders]

Hi!

I also had terrible MPG on my 91 325i.
I fixed alot of things (TPS, Lambda sensor, new hoses) but the biggest culprit were absolutely filthy injectors.

Basically the spray pattern was not correct due all the gunk, so a lot of fuel was wasted.

I ran them in a ultrasonic cleaner, sprayed insides with a carb cleaner. Also tested the relays and the spray patter with a 9v battery. Finally got a rebuild kit to change the seals and gaskets.

Now the car has more power response and the fuel consumption dropped considerably.

So worth checking them.

[Brianmoooore]

If it's an early 320, then idle speed problems are almost always as a result of the weird air slide valve.
Lambda sensors are fitted to cars to protect a cat. from overheating as a result of excess fuel. If a car doesn't have a cat. in the exhaust, then it won't have a lambda sensor.
It's a very primitive electronic injection system, so it's always going to struggle to get fueling spot on, but fuel consumption should be better than 17 MPG (using speedtouch's calculation).
Injectors long overdue for a service is likely to be a contributing factor, as said above, and as I've written on here on multiple occasions. I'd recommend a professional rebuild though, as a DIY'er is unlikely to be able to check and balance the flow rates.
Spark plugs are a useful indicator of fuel mixture ratio, so take them out and post pics. of their working ends.
High idle will, of course, be detrimental to fuel consumption, and don't overlook the possibility of a simple, straightforward leak in the fuel system somewhere.

[Bonymaenjack]

If I may although mine is a 2.5 , my consumption is quite poor as well OBC readings around town are averaging 18.9 mpg ,
From cold the idle is slightly up and down and frequently the engine stalls but is fine when warmed up .

[Satan]

I had my injectors cleaned by Injector Tune who is on here and offers a discount, did it make a difference hmmmmm maybe as I drive like a hooligan anyways and to be honest I don't worry about the mpg as its better than my daily Twinscroll Subaru......

[BristolE30]

Is it an auto? Will get slightly worse from my experience. Sounds like your from US with your 80F days, might be worth finding a local place that cleans injectors like Brian said, as Injectortune is UK based. I’m sure if you fix the dodgy idle your fuel economy will follow suit. Maybe you should change your name to fueldestroyer